Fulvio Cacace

Displacement of a nitro-group by [18F] fluoride ion. A new route to aryl flurides of high specific activity

Nucleophilic displacement of activated nitro-group by [18F] fluoride ion is an efficient route to 18F-labelled aromatics; these compounds can be in the no-flurine-carrier-added state if required.

Experimental Detection of Tetraoxygen

Although suggested by Lewis in 1924 and theoretically predicted, O4 has so far proved extremely elusive, defying all attempts at the positive experimental detection of a bound, intact O4 species. The search has now been brought to an end by the conclusive proof, achieved by neutralization-re-ionization mass spectrometry, of the existence of intact O4 as a gas-phase species with a lifetime in excess of 1 μs, whose dissociation requires overcoming a barrier of the order of 10 kcal mol-1 .

Gaseous Carbonium Ions from the Decay of Tritiated Molecules

Publisher Summary This chapter discusses information on solution chemistry of carbonium ions, and extensive and sophisticated research on their role as intermediates in a variety of organic reactions. The mass spectrometer has gradually developed into a versatile and sophisticated tool for the study of ion-molecule reactions, and provided most of the information at present available on the chemistry of gaseous carbonium ions. A promising approach for introducing carbonium ions of known structure into gaseous systems at any desired pressure, and for studying their reactions by the usual methods of physical organic chemistry, including the isolation and the analysis of the final products, is provided by a technique based on a nuclear transformation. A number of experimental investigations, carried out with specially developed mass spectrometric techniques, confirmed and extended the theoretical predictions, and demonstrated the actual formation of carbonium ions from the decay of T atoms contained in simple organic molecules. The results obtained from the study of the reactions of carbonium ions directly formed from the decay of tritiated molecules in organic systems at normal pressure. The chapter also deals with the reactions of the carbonium ions formed from the protonation of organic substrates with a very strong Brosted acid, the helium tritide molecular ion, obtained from the 8-decay of molecular tritium.

Radiometric Analysis of Tritiated Organic Compounds by Means of Vapor Phase Chromatography

An analytical method, involving the gas chromatographic separation and the quantitative measurement of tritiated volatile compounds, was studied. The method has been successfully employed to detect traces of carrier-free tritiated substances.

Cationic aromatic silylation in the gas phase

Abstract Aromatic substitution by SiMe+3 ions has been studied in the gas phase with an integrated approach, based on mass spectrometry and on a radiolytic technique, characterized by an extended pressure range up to 720 torr and by the actual isolation of the neutral end products, which provides positive structural information. The results demonstrate the occurrence of aromatic substitution, leading to the formation of a CSi bond, and suggest a σ-complex (Wheland intermediate) structure of the ArHSiMe+3 adducts previously detected by ICR and high-pressure mass spectrometry. The selectivity of the silylation, e.g. kT/kB = 3.8 ± 0.4, 14% meta and 86% para orientation in toluene, characterize the reaction, unprecedented both in the gas phase and in solution as a typical electrophilic substitution dominated by the steric requirements of the bulky SiMe3 group, similar in many respects to aromatic alkylation by gaseous t-Bu+.

Nucleophilic aromatic substitution; kinetics of fluorine-18 substitution reactions in polyfluorobenzenes. Isotopic exchange between 18F− and polyfluorobenzenes in dimethylsulfoxide. A kinetic study

Abstract Nucleophilic aromatic substitution reactions of C 6 F 6 , C 6 XF 5 (XH, Cl, Br), C 6 H 2 F 4 , oC 6 H 4 FNO 2 and pC 6 H 4 FNO 2 by the fluoride anion were studied in DMSO utilizing fluorine-18. Substitution of fluorine by fluorine-18 is the only reaction observed with a C 6 F 6 substrate. With other substrates fluorine substitution is predominant. The kinetic studies provide results consistent with a S N Ar two step mechanism and suggest an intermediate analogous to that for electrophilic aromatic substitution. Consideration of σ − indicates consistency with methoxide ion substitution results with similar substrates. The possible utility of these reactions in labeling aromatic compounds is noted.

Bile salt aggregates in the gas phase: an electrospray ionization mass spectrometric study.

Helical and ordered structures have previously been identified by X-ray diffraction analysis in crystals and fibers of bile salts, and proposed as models of the micellar aggregates formed by trimeric or dimeric units of dihydroxy and trihydroxy salts, respectively. These models were supported by the results of studies of micellar bile salt solutions performed with different experimental techniques. The study has now been extended to the gas phase by utilizing electrospray ionization mass spectrometry (ESIMS) to investigate the formation and the composition of aggregates stabilized by noncovalent interactions, including polar (ion-ion, ion-dipole, dipole-dipole, hydrogen bonding etc.) and apolar (van der Waals and repulsive) interactions. The positive and negative ESIMS spectra of sodium glycodeoxycholate (NaGDC), taurodeoxycholate (NaTDC), glycocholate (NaGC), and taurocholate (NaTC) aqueous solutions, recorded under different experimental conditions, show in the first place that aggregates analogous to those present in micellar solutions do also exist in the gas phase. Furthermore, consistently with the condensed-phase model, the positive-ion spectra show that the trimers are the most stable oligomers among the aggregates of dihydroxy salts (NaGDC and NaTDC) whilst the dimers are the most stable among the aggregates of trihydroxy salts (NaGC and NaTC). Moreover, the binding energy of the constituent glycocholate salt units in most gaseous oligomers exceeds that of the corresponding taurocholate units. The ESIMS evidence has been confirmed by vapor-pressure measurements performed on NaGC and NaTC crystals and NaGDC and NaTDC fibers, the results of which show that the evaporation enthalpy of glycocholate exceeds that of taurocholate by some 50 kJ mol(-1).

Charged and neutral NO3 isomers from the ionization of NOx and O3 mixtures.

Mass spectrometric techniques have been utilized in conjunction with theoretical methods to detect and characterize new species formed upon ionization of gaseous mixtures containing ozone and an NOx oxide. NO5+ as well as isomeric NO4+ and NO3+ ions have been identified. Moreover, utilization of neutralization reionization mass spectrometry (NRMS) has provided strong evidence for, if not a conclusive demonstration of, the existence of a new NO3 isomer, in addition to the long-known trigonal radical, as a gaseous species with a lifetime in excess of approximately 1 microsecond.

From N2 and O2 to N4 and O4: Pneumatic Chemistry in the 21st Century

A brief account is given of the theoretical and experimental endeavors that span the last decade and eventually led to the discovery of N(4) and O(4), which have been actively sought for their fundamental interest and their relevance to atmospheric sciences and the development of high energy density materials (HEDM). The discovery and characterization of N(4) and O(4) as metastable species with lifetimes exceeding 1 micros in the isolated gas state have finally been achieved utilizing neutralization-reionization mass spectrometry (NRMS), a powerful technique based on collisional redox processes. The principles of NRMS, experimental outline, main features, and limitations are succinctly examined in comparison with other current approaches to the detection of metastable, short-lived neutral species.

Gas-phase reactions of nitronium ions with acetylene and ethylene: an experimental and theoretical study

A comparative study of the gas-phase reactions of NO2+ with acetylene and ethylene was performed by using FT-ICR, MIKE, CAD, and NfR/ CA mass spectrometric techniques, in conjunction with ab initio calculations at the MP2/6-31+G* level of theory. Both reactions proceed according to the same mechanism, that is, 1,3-dipolar cycloaddition, but yield products of different stability. The C2H2NO2+ adduct from acetylene has an aromatic character and hence is highly stabilized with respect to the C2H4NO2+ adduct from ethylene. Both cycloadducts tend to isomerize into O-nitroso derivatives, that is, nitrosated ketene and nitrosated acetaldehyde, which represent the thermodynamically most stable products from the addition of NO2+ to acetylene and ethylene, respectively. As prototypal examples of the reactivity of free nitronium ions with most simple pi systems, the reactions investigated are useful starting points to model the mechanism of aromatic nitration.